Mr. Toiilmiii Smith on the Ventriculidse of the Chalk. 279 
chyma, it appears to me that the laws of growth of leaves must 
he looked for in the course of the development of their frame- 
work, the nerves. These are apparently organized gradually out 
from the stem into the nascent leaves^ just as the vascular bundles 
into the apex of the stem, and their point of separation in the 
blade being fixed from the first, it is clear that all growth in the 
blade of the leaf must occur beyond this, and it is most natural 
to suppose that the nerves become organized from this centre 
outward as the vascular bundles were from the stem at first. 
Thus it would happen that Dicotyledonous leaves in general 
would grow at their base until they were sketched out as it were, 
in the bud, but as soon as the nerves were formed and the plan 
of the framework of the future expanded lamina laid down, the 
growth would be apical, marginal and interstitial. In Monoco- 
tyledonous leaves with straight veins there appears to be nothing 
to prevent the continued development of the base, and as we 
usually find the tissue in a softer and less consolidated condition 
there, it is probable that that part is the seat of development. 
These ideas are merely suggested as rational interpretations of 
the facts before us, but much systematic observation is required 
before this question can be settled. 
XXIX. — On the Ventriculidse of the Chalk; their classification. 
By J. Toulmin Smith, Esq. 
[Continued from p. 220.] 
Genus Cephalites. 
Character. Pouch-shaped : very constant in size and dilatation : 
cavity usually regular and with a single opening ; sometimes 
winding and with more openings than one : membrane form- 
ing the wall of the cavity always deeply folded : marginal 
edges — and, sometimes, most prominent points — of the plaits 
attached to a simple apolypiferous membrane stretched across 
their whole breadth and forming the upper margin or head of 
the wall : membrane of wall polypiferous on both external and 
internal surfaces. 
The difierences between the genera Cephalites and Ventriculites 
are so broadly marked that, except in one or two species, it would 
be difficult to confound even fragments of the two. In every 
species of Cephalites the head is conspicuous and unmistakeable. 
This very remarkable peculiarity is alone sufficient to distinguish 
the genus*. 
* See ante, p. 46. 
